This invention relates generally to imaging and, more particularly, to emissions tomography imaging.
At least some known x-ray or nuclear “photon counting” tomography systems have limited count capabilities due to a width of an analog detection signal (about 150-200 nanoseconds or more). The width of the analog signal generally depends on the detector used. An x-ray detector may have a signal width of approximately 150-200 nanoseconds, but a Sodium Iodide (NaI) scintillation crystal, used in gamma cameras, may produce temporally longer pulses than a Bismuth Germanate (BGO) scintillation crystal commonly used in positron emission tomography (PET). In relatively high-count rates, individual pulses representing individual detection events may “pile-up” or arrive at detection circuitry at a rate that exceeds the counting capability of the detection circuitry, for example, a comparator may not have time to return to a low or zero level before a next pulse arrives. Accordingly, a plurality of pulses may be counted as a single event.
At least some known imaging systems use count rate correction methods to attempt to accurately determine pulses due to each individual detection. A function of a “true count rate” vs. a “measured count rate” may be found experimentally, for example, using a strong radioactive source with a known decay time and measuring the count rate over a long duration, or may be calculated from a theoretical model of the detector, trigger, and counter system. However, such methods only statistically correct the count rate and consequently add noise to the signal. Such methods do not recover the lost count.
For example, for a random distribution of pulses, if the statistical noise associated with N counted pulses is N1/2, the Signal to Noise Ratio (SNR) may be shown to be 1/N1/2. Assuming a non-buffered triggering system, at true rate, T, pulses per second, the measured rate will be T*e−2*T/R, where R is the characteristic rate of the system. When the true rate, T is relatively low, T<<R, then the measured rate (M) is given by M=T*e−2*T/R˜T For relatively higher count rates, the equation has to be solved to yield T from knowing M, from, for example, a lookup table. However, the SNR associated with the calculated T is at least as large as 1/M1/2, which is worse than 1/T1/2.